An automatic transmission for a vehicle is provided with a hydraulic control circuit that supplies hydraulic pressure (oil) used for driving and/or cooling the transmission (for instance, see Patent document 1).
Referring now to FIG. 5, there is shown an explanatory drawing illustrating part of the essential circuit configuration (essential circuit components) of a hydraulic control circuit of an automatic transmission for a vehicle of a conventional example.
A hydraulic control circuit 400 of an automatic transmission for a vehicle shown in FIG. 5a is provided with a pressure adjusting valve 410 for adjusting a discharge pressure of an oil pump OP to a line pressure P1. A pressure regulating valve 420 is provided downstream of the pressure adjusting valve 410, for regulating a supply pressure (torque converter pressure P2) to a torque converter T/C (not shown).
In the hydraulic control circuit 400, a flow control valve 430 is also provided in an oil passage 500 that connects the oil pump OP and the pressure adjusting valve 410, for adjusting the flow of oil to the pressure adjusting valve 410. The flow control valve serves to return part of oil discharged from the oil pump OP back to the upstream side of oil pump OP during driving of oil pump OP at high rotational speeds, for preventing excessive hydraulic pressure from being supplied to the pressure adjusting valve 410.
The aforementioned flow control valve 430 has a throttle (a flow constriction) 431 provided in the oil passage 500, a bypass passage 432 that connects the upstream side and downstream side of throttle 431, while bypassing the throttle 431, and a connecting passage 434 that connects the bypass passage 432 and the upstream side of oil pump OP. When an excessive hydraulic pressure acts on the flow control valve 430, a valve element 435, which is disposed in the bypass passage 434, moves to a position at which the bypass passage 432 and the connecting passage 434 are communicated with each other. Thus, part of the hydraulic pressure acting on the flow control valve 430 is returned back to the upstream side of oil pump OP, for preventing the excessive hydraulic pressure from being supplied to the pressure adjusting valve 410.
By the way, with the throttle 431 provided in the oil passage 500, the throttle 431 serves as a flow resistance that impedes the flow of oil. As a result, the load on the oil pump OP increases, thereby deteriorating fuel economy. For the reasons discussed above, in recent years, there have been proposed various hydraulic control circuits in which a flow control valve 430 has been omitted or eliminated (see FIG. 5b).
However, assuming that the flow control valve 430 has been eliminated, there is a tendency for excessive hydraulic pressure to be supplied to the pressure adjusting valve 410 during driving of oil pump OP at high rotational speeds. In this case, the pilot pressure P3, which is supplied from the pressure adjusting valve 410 to the pressure regulating valve 420, also tends to increase (see FIG. 5b).
By way of the pressure regulating valve 420, the drain amount of oil drained from the pressure regulating valve 420 is adjusted, thus regulating a hydraulic pressure (torque converter pressure P2) supplied to the torque converter side. However, as the pilot pressure P3 acting on the pressure regulating valve 420 increases, the torque converter pressure P2 also increases. Assuming that the increased torque converter pressure P2 reaches a withstand pressure of the torque converter or more, the increased torque converter pressure affects the durability of the torque converter.
In such a case, for the purpose of a drop of hydraulic pressure inputted to the pressure regulating valve 420, a throttle (a flow constriction) may be provided between the pressure adjusting valve 410 and the pressure regulating valve 420. However, the pressure regulating valve 420 utilizes the hydraulic pressure inputted from the pressure adjusting valve 410 as a drive pressure P4 of the pressure regulating valve 420 (see FIG. 5b). Hence, assuming that the pilot pressure P3 inputted into the pressure regulating valve 420 has been simply dropped without much deliberation, there is a possibility that the operation of pressure regulating valve 420 cannot be appropriately performed. In this case, the torque converter pressure P2 cannot be adjusted to an appropriate pressure.
Therefore, it would be desirable to permit the torque converter pressure to be adjusted to an appropriate pressure even when a flow control valve has been eliminated.